This invention relates to a drain valve and, more particularly, to a drain valve which is especially useful for draining liquid from a closed vessel whose interior is under a vacuum at the time the valve is first opened. One such vessel forms part of a fuel/water separator or filter for an internal combustion engine. As fuel flows through the filter element, water is separated therefrom and collects at the bottom of the vessel. Periodically, water and other contaminants must be drained from the vessel. Rapid and complete draining can be effected only if ambient air is admitted into the vessel to break the vacuum therein.
Drain valves which are particularly adapted for use with fuel/water separators are disclosed in Wilson U.S. Pat. No. 4,314,689 and Matheson U.S. Pat. No. 4,440,193. In those valves, the main sealing ring between the valve member and its housing moves with the valve member and out of the housing when the valve member is opened and thus the sealing ring is exposed to external contamination such as grease and dirt.